The present invention relates to a photochemically catalyzed process for controlling the sol-gel process, with which structured or non-structured stress-free coatings and shaped articles can be produced, in particular those which can be converted into glasses or ceramics purely by heat treatment.
Hydrolytic polycondensation by the sol-gel process is a process which is well-known in the prior art for production of amorphous inorganic networks which can be converted into glasses, ceramics and inorganic-organic hybrid materials (see, for example, H. Schmidt, J. Non-Crystall. Solids, 100 (1988), page 51, C. J. Brinker, loc. cit. page 31 and J. Fricke. loc. cit. page 169).
Monomeric or precondensed hydrolyzable and condensable compounds having an inorganic central atom are hydrolyzed and precondensed by adding water and if appropriate catalyst, until a sol forms; condensation to a gel is then carried out, usually by adding a pH-active catalyst. The gel can be converted into the abovementioned materials by treatment with heat and/or radiation.
For many uses, such as, for example, in microelectronics and optoelectronics and in the production of optical materials and fiber materials, a controlled initiation or reaction procedure in the sol-gel process and structuring of the material produced thereby is indispensable. A non-structured retarding influence is required for stress-free production of coatings and massive shaped articles.
Control of the course of the reaction (hydrolysis and condensation reactions to give the inorganic network) was previously possible only by changing the reaction temperature and/or adding catalysts. These two possibilities of control have the following disadvantage:
Temperature control:
On heating up, which accelerates the sol-gel process, and on cooling, which retards the process, temperature gradients are formed which cause an uncontrolled reaction gradient. Heating up is limited by the boiling or evaporation properties of the solvents of the sol-gel process. High spatial resolution of the heat supply, such as is necessary for structuring a shaped article or a coating, cannot be achieved because of the thermal conduction.
Catalysts:
Catalysts for accelerating or retarding the sol-gel process have an effect immediately after they are added, that is to say an acceleration or retardation of the reactions of the sol-gel process already starts before thorough mixing is achieved. Uncontrollable inhomogeneities develop as a result. This behavior also prevents the use of these catalysts for coatings in which the crosslinking of the inorganic constituents is to take place only on the substrate. Controlled structuring is not possible by means of this process.
The only possibility of effecting structuring in coatings which have been prepared by the sol-gel process hitherto consisted in introducing photochemically active organic substituents into the starting compounds and irradiating these or prepolymers thereof in a structuring manner. These processes have been used for material coatings which are to be structured. However, the introduction of these photochemically active substituents limited the temperature stability of the sol-gel materials considerably.
Another problem which can be overcome only with difficulty using the prior art is selective initiation of the polycondensation of highly reactive compounds of ceramic-forming elements. The addition of water, which must be performed very carefully, to avoid premature precipitation of the hydroxides, is carried out in this case by processes which are difficult to control, such as addition of carriers charged with moisture or by absorption from an atmosphere charged with moisture.